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<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
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  <title>Document</title>
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<body>
  <script>
    var arr=[1,2,3,4,3,2,1];
           //0 1 2 3 4 5 6
    document.write(
      arr.indexOf(3)+"<br>"+//2
      arr.indexOf(3,3)+"<br>"+//4
      arr.indexOf(3,5)+"<br>"+//-1
      arr.indexOf(5)//-1
    );
  </script>
</body>
</html>